This invention relates to systems for transmitting digital signals. More particularly, it is concerned with systems for driving and terminating a transmission line.
Conventional high speed connections on printed circuit boards, between cabinets, or on communication links employ driver circuitry at the transmitting end and require terminations at the receiving end in order to preserve the quality of the digital signal being transmitted. Conventional termination schemes dissipate power, and in low power CMOS circuits the terminations are the major cause of power dissipation.
FIGS. 1A and 1B illustrate, in equivalent circuit diagrams, two different schemes in the prior art which are conventional for terminating transmission lines driven by driver circuitry at the transmitting end. In the circuit of FIG. 1A when, in effect, switch SA1 is closed and switch SA2 open, the voltage on the transmission line 10 becomes +5 volts. With the received voltage V.sub.OUT +5 volts, power is dissipated in the termination resistance RA1. When switch SA1 is open and switch SA2 closed, the voltage on the transmission line 10 becomes 0 volts. With the output voltage of the transmission line 10 V.sub.OUT 0 volts, no power is dissipated at the termination.
FIG. 1B illustrates an alternative conventional prior art scheme in equivalent circuit form. When, in effect, switch SB1 is closed and switch SB2 is open, the voltage V.sub.OUT at the receive end of the transmission line 11 is +5 volts and no power is dissipated at the termination. When switch SB1 is open and switch SB2 is closed to produce a voltage V.sub.OUT of 0 volts at the receive end of the transmission line 11, power is dissipated in the termination resistance RB1.